1. Field of the Invention
The present invention relates to an optical displacement detection mechanism in which a light from a light source is irradiated to a measurement object to thereby detect an intensity of the light after the irradiation by a photodetector, and especially, to a surface information measurement device, such as scanning probe microscope, surface roughness meter, hardness meter or electrochemical microscope, which measures, by using this optical displacement detection mechanism, a shape information and various physical information (e.g., dielectric constant, magnetized state, transmissivity, viscoelasticity, friction coefficient, and the like) of a sample surface.
2. Description of the Related Art
A scanning probe microscope (SPM: Scanning Probe Microscope) is known as a device for performing observations on irregularities information of the sample surface, a-physical property information and the like by measuring in a micro region of a sample of metal, semiconductor, ceramic, resin, high polymer, biomaterial, insulator and the like.
In the scanning probe microscope, one possessing a sample holder on which the sample is mounted, and a cantilever which has a probe in its tip and is approached to a surface of the sample, becomes well known. And, there is made such that a surface shape and the various physical property information are measured by performing a distance control between the sample and the probe by relatively scanning the sample and the probe in a sample face (XY plane) and operating the sample or the probe in a direction (Z direction) intersecting perpendicularly to the sample surface while measuring during this scanning a displacement quantity of the cantilever by a displacement detection mechanism.
FIG. 6 shows a schematic, view of the structure of the scanning probe microscope in which a conventional, typical optical displacement detection mechanism is used (e.g., refer to JP-A-10-104245 Gazette).
A scanning probe microscope 201 of FIG. 6 has a sample stage 212 where a sample 211 is mounted to face a tip and, by a three-axis minute movement mechanism (scanner) 213 constituted by a cylindrical piezoelectric element whose terminal is fixed onto a base 215, the sample 211 is minutely moved in the direction (Z direction) perpendicular to the sample face while being scanned in the sample face (XY plane).
Further, a cantilever 207 having in its tip a probe 209 is retained to a support post 203 fixed to a base 215, through an arm 205 whose rigidity is high. It is a constitution in which, in a tip part lower face of the cantilever 207, the probe 209 is formed so as to protrude downward, and a tip of the probe 209 is approached to the sample 211 surface by a rough movement mechanism (not shown in the drawing) capable of operating in the Z direction.
Above the cantilever 207, there is provided an optical displacement detection mechanism constituted by a semiconductor laser (LD) 221 and a photodetector 235 whose material is made from a semiconductor, and generally called an optical lever system.
Here, an operation principle of this optical displacement detection mechanism of the optical lever system is detailedly explained. (For example, refer to Non-Patent Document 1: Takeshi Fukuma et al., Development of low noise cantilever deflection sensor for multienvironment frequency-modulation atomic force microscopy, REVIEW OF SCIENTIFIC INSTRUMENTS, 76, 053704 (2005))
FIG. 7A is a schematic constitutional view of an optical displacement detection mechanism 200, and FIG. 7B is an electric circuit view connected to the photodetector 235 whose material is made from the semiconductor. In this optical displacement detection mechanism 200, a laser light (incident light 231) from the light source 221 disposed above the cantilever 207 and comprising a semiconductor laser is condensed and irradiated to a back face of the cantilever 207 by a lens 240. This incident light 231 reflects in the back face of the cantilever 207, and a reflected light 233 impinges against the photodetector 235 obliquely disposed above the cantilever 207 and constituted by the semiconductor. This photodetector 235 is a constitution in which its light reception face is bisected upward and downward, and can detect an incident position of the reflected light 233.
By measuring an intensity difference between the lights entering to a region A of an upside light reception face and a region B of a downside light reception face of the photodetector 235 as shown in FIG. 7B, it becomes possible to measure a deflection quantity of the cantilever 207. If the lights enter to the photodetector 235, light signals are converted into electric signals, and electric currents iA and iB generate from the respective light reception faces A and B. These electric currents are converted into voltage signals VA an VB by an electric current/voltage conversion circuit 242 constituted by an operational amplifier 245 and a feedback resistance R1V, which are connected to the respective light reception faces. At this time, if a feedback resistance value of the electric current/voltage conversion circuit 242 is R1V, there are relations of VA=R1V×iA, and VB=R1V×iB. Like this, the electric current/voltage conversion circuit 242 acts as an initial stage amplifier converting an electric current signal into the voltage signal at an amplification rate R1V.
These voltage signals VA and VB are sent to a differential amplification circuit 243 constituted by an operational amplifier 246 and resistances R2 and R3, and detects a difference signal VA-B of the voltage. Here, like FIG. 7A and FIG. 7B, in the case where the differential amplification circuit is constituted by the operational amplifier and the resistance values R2 and R3, a relation of VA-B=(R3/R2)×(VA−VB) is effected, and the differential amplification circuit 243 acts as an amplifier amplifying the voltage signal at an amplification rate R3/R2, thereby outputting the voltage signal VA-B.
Here, in FIG. 6 and FIG. 7A and FIG. 7B, in a case where the probe 209 and the sample 211 approach relative to each other, an interatomic force acts first and, if they are additionally approached, a contact force acts, so that a deflection occurs in the cantilever 207. If the cantilever 207 deflects, a spot 241 on the light reception face of the photodetector 235 moves upward and downward. Here, by detecting the voltage signal VA-B of the difference between the upper and lower light reception faces by the differential amplification circuit 243, it becomes possible to measure the deflection quantity of the cantilever 207. Incidentally, in order to cut a frequency component other than a band used in the measurement to thereby suppress a noise, a band-pass filter 244 is normally provided in a back side of the differential amplification circuit 243, and a signal passing through this band-pass filter 244 is sent to a Z feedback circuit 251.
Since the deflection quantity of the cantilever 207 depends on a distance between surfaces of the probe 209 and the sample 211, an irregularities image of the sample surface is obtained by detecting the defection quantity of the cantilever 207 based on the output voltage VA-B of the photodetector 235, inputting it to the Z feedback circuit 251, controlling the distance between surfaces of the probe 209 and the sample 211 by the Z minute movement mechanism 213 such that the deflection quantity becomes constant, i.e., the output voltage VA-B becomes constant, and scanning the sample by the XY scanner 213. These controls are performed by a control section 257, and the three-axis minute movement mechanism 213 is driven by an XYZ scanner driver 253. The obtained irregularities image is displayed to a display section 255.
In this optical displacement detection mechanism, a resolving power of a measurement data in a height direction is determined by magnitudes of a detection sensitivity (output voltage quantity per unit length) of the displacement detection mechanism and a noise component mixed in the signal of the optical displacement detection mechanism.
Here, as factors of the noise in the optical displacement detection mechanism, several reasons are considered (refer to Non-Patent Document 1: Takeshi Fukuma et al., Development of low noise cantilever deflection sensor for multienvironment frequency-modulation atomic force microscopy, REVIEW OF SCIENTIFIC INSTRUMENTS, 76, 053704 (2005)).
(1) A shot noise of the photodetector
(2) A Johnson noise (thermal noise) of the photodetector
(3) A quantum noise of the light source
(4) A returned light noise and a mode hop noise of the light source
(5) A thermal fluctuation of the cantilever
(6) An interference noise of the light
Among these, one in which a degree of dependence is highest in a frequency band used in the normal scanning probe microscope is the shot noise of the photodetector of (1), and a ratio of the shot noise to the detection sensitivity becomes small in inverse proportion to a square root of a light quantity P in the light reception face.
Further, if there becomes a region in which the frequency at the measurement time is high, it follows such that also the degree of dependence of the Johnson noise of (2) increases, and a ratio of the Johnson noise to the detection sensitivity becomes small in inverse proportion to the light quantity P in the light reception face.
Here, if there are supposed that the light quantity P in the light reception face is an output PO of the light source, and a transmission efficiency of the light on an optical path from the light source to the photodetector via a measurement object is α, there is expressed as P=αPO
Like this, as to the shot noise and the Johnson noise, if the intensity P on the light reception face of the photodetector increases, a quantity of the noise in regard to the detection sensitivity decreases and, as a result, a resolving power of the measurement data rises. That is, enlarging the output PO of the light source or rising the transmission efficiency on the optical path is effective for lowering the ratio of the noise to the detection sensitivity.
On the other hand, if there is considered about the noise in a light source side of a semiconductor laser that is the light source most generally used in the conventional optical displacement detection mechanism, as to the semiconductor laser, in its low power region, a ratio of a natural emission light in an element inside becomes many, and a noise called the quantum noise of (3) generates. As a power is raised, a ratio of an induced emission light becomes predominant, and a ratio of the quantum noise decreases. However, as to the semiconductor laser, the quantum noise exclusively decreases as the output is enlarged and, in a case where there is driven by a high output, there generate, as shown in (4), the returned light noise which returns to the semiconductor laser by reflecting in the cantilever and the sample or the optical element etc. disposed in the optical path, and the mode hop noise generating at a fluctuation time of a temperature or the light output. Therefore, an optimum value exists in the output in a light source side, and a drive is performed below 2 mW in the prior art. Like this, in order to lower a quantum noise level of the photodetector, although it is necessary to enlarge the output in the light source side, there is a limit in suppressing the generations of the returned light noise and the mode hop noise in the light source side.
Further, in order to reduce the mode hop noise and the returned light noise, it is effective to lower a coherency of the light source. In other words, in a spectrum of an intensity in regard to a wavelength of the light source, it is desirable to use a light source in which a spectrum width of a portion, in which the intensity becomes maximum, is wide and, with this purpose, a high frequency modulation is applied to the semiconductor laser. Further, in order to prevent the returned light by the measurement object, a member on the optical path, and the like, there is performed such a contrivance as to use an optical system in which the reflected light does not return to the semiconductor laser by changing polarized light states of the incident light and the reflected light. However, even if such a contrivance is performed, since the mode hop noise and the returned light noise cannot be eliminated completely, the drive is performed with a light intensity in the light source side being made below 2 mW.
Further, since the semiconductor laser is the light source in which the coherency is high and which is excellent in an interference possibility, in the scanning probe microscope for instance, the reflected light in the cantilever interferes with a light reflecting from the sample while overflowing the cantilever, so that there is a case where the interference noise of (6) occurs in the irregularities image and the data at the physical property measurement time in regard to the distance between the probe and the sample.
Like the above, although the optimization of the output of the light source is performed such that a ratio of the noise to the detection sensitivity of the optical displacement detection mechanism becomes small by optimizing the output of the light source, in the conventional optical displacement detection mechanism, since the light intensity in the light source side and an amplification rate of the amplifier in the photodetector side are fixed, there are such issues as mentioned below.
(1) By optical characteristics, such as reflectivity, of the measurement object and a shape of the measurement object, since the light intensity in the light reception face of the photodetector changes, the detection sensitivity and the ratio of the noise differ by the measurement object.
(2) There is a case where, by an irradiated light to the measurement object from the light source, the measurement object is heated, and thus the measurement object deforms.
(3) In the optical displacement detection mechanism of the optical lever system, or the like, the detection sensitivity changes by a shape such as length and mechanical characteristics such as spring constant of the measurement object.
Especially, in the scanning probe microscope, there is performed such a contrivance as to earn the reflectivity by coating aluminum, gold or the like to the cantilever made of silicon or silicon nitride, which is the measurement object, in order to prevent a lowering of the detection sensitivity and an increase in the noise. Further, in a case where electrical characteristics of the sample are measured by the scanning probe microscope, the cantilever is caused to have an electrical conductivity by coating an electric conductor such as gold and rhodium to the cantilever. Further, depending on the sample to be measured, there is selected the cantilever in which the mechanical characteristics, such as spring constant and resonance frequency, and the shape differ.
Accordingly, in the scanning probe microscope, the reflectivity greatly differs depending on the cantilever, and the detection sensitivity and the ratio of the noise differ depending on the measurement object.
Further, by a difference in linear expansion coefficient between a material of a film coated to the cantilever and a base material of the cantilever, a temperature of the cantilever rises by the irradiated light from the light source, so that the cantilever generates a deflection deformation. In a case where there is considered the ratio of the noise to the detection sensitivity, since the light is irradiated to the cantilever at a high intensity, an influence of the thermal deformation is large in the cantilever whose spring constant is small.
Further, depending on a selected different shape of the cantilever, the detection sensitivity changes since a lever ratio of the optical lever changes.
By the issues like the above, in the conventional optical displacement detection mechanism, a measurement accuracy deteriorates.